1. Field of the Invention
The present invention generally relates to optical scanning systems, and more particularly to an optical scanning system having a laser beam source, a deflector and hologram elements and performing highly-precise laser scan.
Optical scanning systems are applied to various devices such as a laser beam printer and an optical instrument, and scan an object by deflecting the laser beam. It is required that these optical scanning systems can realize a highly-precise laser scan.
2. Description of the Prior Art
An optical scanning system having a polygon mirror and an f-.zeta. lens system is well known as an optical scanning system in which the laser beam scans an object.
However, such an optical scanning system has a disadvantage in that it is very expensive because the f-.THETA. lens system is made up of a plurality of lens groups. With the above in mind, recently, there has been considerable activity in the development of an optical scanning system that uses a hologram disk. As shown in FIG. 1, an optical scanning system using a hologram disk utilizes a characteristic in which the diffracting direction of the incident beam depends on the position of the incident beam on a lens. Recently, it has been proposed that a plurality of hologram elements are used instead of a lens.
FIG. 2 shows such an application of hologram elements, in which a hologram disk is illustrated. The hologram disk includes a plurality of hologram elements disposed on a disk in a ring formation. The hologram disk is rotated to move the incident laser beam that scans an object.
Optical scanning systems using hologram disks as described above have a disadvantage in that the image plane of the laser beam is moved due to a variation in the wavelength of the laser beam. In order to eliminate the above disadvantage, other hologram elements are additionally used to compensate for a variation in the wavelength of the laser beam to make the laser beam project onto an identical point even when the wavelength of the laser beam varies.
The optical scanning system using the above-mentioned hologram disk realizes highly-precise laser scan. However, the hologram disk does not have strong mechanical strength, and cannot be rotated at high speeds. Hence, the optical scanning system using the hologram disk cannot realize high-speed laser scanning.